Point-of-use filtration

The negative affect on yield rates and tool uptime of acidic, basic and organic molecules in the gas phase is receiving increasing attention within the semiconductor industry. While in deep ultra violet (DUV) lithography, the filtration of resist-poisoning amines has been carried out for a number of years, newer concerns also include the degradation of optics, chemical contamination of reticles, changes in resist sensitivity etc. This all contributes to the evolution of air and gas purity requirements within exposure tools. In other areas, such as mask making, the use of chemical filtration has also become necessary due to the implementation of chemically amplified resists (CAR) and DUV wavelengths. Using a controlled environment at the point-of-use, the OEM or the toolowner can specify the type of environment required locally. This offers benefits in terms of tool uptime and yield rates, and also, by limiting the size of this environment, the operation costs can be substantially reduced. The advantages of point-of-use filtration are numerous:

• The supply of high purity air or gas is restricted to the targeted critical area.
• Adapted solutions can be developed for existing designs, making it an ideal tool for retrofit situations.
• Cost of ownership is significantly reduced, since the flow with the tightest specifications is reduced to a minimum.
• Maintenance is spaced out, since a reduced flow also means an increased filter lifetime.

Humidity levels In high relative humidity conditions, physical adsorption of activated carbons is often reduced, the adsorption of water having a negative impact on its capacity for organics. This must be compensated for by careful selection of carbon material or modifications to the carbon surface. For the removal of basic and acidic species, high relative humidity levels enhance the chemisorption filtration mechanism of chemically impregnated carbon, while at extremely low levels of water content (dew point <-70°C) removal efficiency is reduced making other media such as ion exchange better suited. For each specific application, it becomes essential to select or combine different media to cover the range of contaminants of concern. Clean dry air used for air bearings in critical process areas, nitrogen or clean air used for purging optical elements in lithography tools are examples of point-of-use filtration that help address today's technology's challenges. Field tests have shown contamination levels of basic, acidic and organic contaminants downstream of these filters to be below detection limits of 0.1ppb. Another area of interest is the protection of reticles (Fig. 2): similarly to disk drive and sensor applications where breathers are used for protection against particulate contamination, moisture or acid gas contaminants, current pellicle frames protecting the reticles are fitted with breather membranes. So far, these membranes are for particulate filtration only while chemical contamination issues of reticles, sometimes causing killer defects, are increasingly observed with the move to 193nm lithography and to 300mm wafers2. A possible solution is a breather membrane ensuring chemically clean, as well as particulate-free air, in the semi-closed environment between reticle and pellicle frame. Solutions similar to those developed for the disk drive industry may be of use.

Conclusion Adopting point-of-use filtration within lithography exposure tools has solved severe contamination problems with efficient and cost-effective solutions. Given the variations in environmental conditions of air or gas flows in different locations, specific combinations may be required for each application. Reticle contamination issues, with breather membrane solutions, are also being investigated. At the cleanroom level, the move from Class 1 to SMIF environments, with tighter air purity requirements, also brings chemical contamination challenges that will have to be tackled with point-of-use solutions.

Paul Vannerem, Donaldson Europe Tel. : +32 16 38 39 83 email: pvannere@mail.donaldson.com